1. Field of the Invention
The present invention relates to a piezoelectric oscillator which outputs a temperature-compensated frequency signal.
2. Description of the Related Art
A crystal resonator, which is a piezoelectric resonator used for a piezoelectric oscillator circuit, for example a crystal oscillator circuit, has frequency-temperature characteristics such that an oscillation frequency varies depending on temperatures. These frequency-temperature characteristics are different among crystal resonators due to cutting angles and thicknesses of crystal pieces constituting the crystal resonators. FIG. 6 illustrates an example of characteristics obtained by a crystal resonator using, for example, an AT-cut crystal piece with respect to such frequency-temperature characteristics. In FIG. 6, the vertical axis of the graph shows a frequency deviation (Δf/f0, Δf=f−f0) between a set frequency f0 as a target (at which oscillation is desired) and a frequency f actually obtained at a temperature T, and the horizontal axis shows the temperature T of the crystal resonator. Symbol T0 denotes a reference temperature, for example 29° C., and a control voltage of the oscillator circuit is set so that the set frequency f0 is obtained at this temperature T0. When oscillation in a thickness shear vibration mode is generated using the AT-cut crystal piece, the frequency deviation Δf/f0 is approximated by equation (1) which is a cubic function of the temperature T.Δf/f0=α(T−T0)3+β(T−T0)+γ  (1)
In this equation (1), symbols α, β, and γ denote constants obtained separately for each crystal resonator, and these constants (α, β, γ, T0, and f0) are obtained by placing the crystal resonator in a thermostatic oven, measuring the frequency f while varying the temperature T, and solving a simultaneous equation obtained by substituting measurement results of the temperature T and the frequency f in equation (1).
Thus, various contrivances are made to stabilize the frequency irrespective of the atmospheric temperature. For example, in a temperature compensated xtal oscillator (TCXO), a control voltage and an output frequency are in a linear relation, and so temperature compensation is performed by providing a temperature sensor, for example a thermistor, in the package of the oscillator, obtaining a compensation voltage based on a temperature detection value from this temperature sensor and a reference temperature, and adding this compensation voltage to a reference voltage (control voltage by which the set frequency f0 can be obtained at the reference temperature T0). However, it is said that in this TCXO, the stability of the frequency-temperature characteristics of the crystal resonator after compensation (adjustment) is, for example, about ±0.2 ppm at the maximum in the temperature range of −20° C. to 75° C., and further improvement in stability is difficult.
On the other hand, as an oscillator that compensates the frequency-temperature characteristics of the crystal resonator besides the above-described TCXO, a D-TCXO (Digital-Temperature Compensated Xtal Oscillator) is known for example. In this D-TCXO, the control voltage V is adjusted by digital control. So, it is conceivable that the stability of the frequency-temperature characteristics can be improved more than by the above-described TCXO, but electronic noise is generated more than in the TCXO.
There is also known an OCXO (Oven Compensated Xtal Oscillator) which controls the frequency-temperature characteristics of the crystal resonator in an analog manner using an oven (thermostatic oven). In this OCXO, the stability of the frequency-temperature characteristics and an electronic noise characteristic are improved more than in the TCXO, but large power is consumed, which makes it expensive.
Further, there is also known an MCXO (Microcomputer Compensated Xtal Oscillator) in which, for example, a signal in a thickness shear vibration mode and a signal in a thickness twist vibration mode, which occur in a pair of electrodes formed on one face of an SC-cut crystal piece, are separated with a filter, the signal in the thickness shear vibration mode is taken out as a main oscillation (set frequency f0), a temperature is detected from the signal of the thickness twist vibration mode, and the frequency-temperature characteristics (control voltage) of the crystal oscillator are controlled using a microcomputer based on this temperature. In this MCXO, the stability of the frequency-temperature characteristics and the electronic noise characteristic can be improved more than in the TCXO. However, since the signals in the two modes are separated with the filter, the circuit structure is complicated and consumes large power, which makes it expensive.
Japanese Patent Application Laid-open No. H7-190916 describes an odorous substance sensor in which one piezoelectric resonator plate is sectioned into plural vibration areas with slits, a pair of electrodes are disposed in each of the vibration areas, and plural odorous substances are recognized by absorbing odorous substances in a synthetic resin film applied on these electrodes. However, the above-described problems are not considered.